The present invention relates to a process for preparing aluminum fluoride and the so obtained aluminum fluoride.
Specifically the invention relates to an improved process for preparing aluminum fluoride starting from the corresponding oxide.
Aluminum fluoride (AlF3) is an inorganic solid well known in the art of heterogeneous catalysts preparation. It is used both as such and as a support for metal salts having catalytic activity due to its strong acidity according to Lewis and/or its thermal and chemical inertia. It is widely used, for example, in the fluorination of chlorinated organic compounds to obtain the corresponding fluorinated compounds; in this case it is used also as a support of chromium, cobalt, nickel salts, etc. See for instance U.S. Pat. No. 4,967,023 and U.S. Pat. No. 5,008,475.
The preparation on an industrial scale of the aluminum fluoride is carried out by fluorination of the aluminum oxide (alumina) with anhydrous hydrofluoric acid (HF). Other methods are also known which however have substantially lab applications, such as the hexafluoroaluminates pyrolysis; the alumina treatment with organic chloro-fluorinated compounds; the F/X exchange on an aluminum halide, wherein X is equal to Cl, Br; etc.
The alumina fluorination with anhydrous HF to obtain AlF3 is well known in the art and is described for example in FR 1,383,927, wherein the so obtained AlF3 is then used as such to catalyze the HF sum on acetylene.
As is well known to those skilled in catalysts preparation, it is desirable that a solid which must be used as heterogeneous catalyst, both as such and as a support for a catalytically active phase, has a surface area (SA) as high as possible. Typical examples of solids used for this purpose are the various types of alumina with surface areas of 200-300 m2/g.
However the alumina fluorination to obtain AlF3 drastically reduces said value: the obtained AlF3 indeed shows lower SA. In order to try to increase this value it is well known to limit fluorination below the stoichiometric.
This process of course cannot be applied to the fluorination catalysts, since the HF used as a reactant would lead to completion of alumina fluorination with consequent loss of surface area and catalytic activity.
In order to overcome this drawback it has been suggested, see GB 1,000,485, to regenerate alumina when it is excessively fluorinated, by treatment with steam at high temperatures. Said process is of course not very practical on an industrial scale, especially due to the great aggressiveness of the gaseous mixture of HF and water at high temperature which is thus produced. A further drawback is that these kinds of catalyst require frequent regeneration and therefore the interruption of the production processes where they are used.
It has now been surprisingly and unexpectedly found by the Applicant that it is possible to obtain AlF3 having an high surface area by alumina fluorination with anhydrous HF by using the process described hereinafter wherein said fluorination is carried out under very specific conditions.
It is therefore an object of the present invention to prepare aluminum fluoride having a high surface area comprising fluorination of alumina with gaseous HF having surface area of at least 150 m2/g, characterized in that the alumina is fluorinated with HF at an initial temperature lower than 300xc2x0 C., preferably in the range 100xc2x0-280xc2x0 C., still more preferably in the range 150xc2x0-200xc2x0 C., the temperature is raised with a temperature gradient xe2x89xa6100xc2x0 C./hour up to the final temperature  greater than 320xc2x0 C. and  less than 450xc2x0 C., preferably in the range 350xc2x0-400xc2x0 C.; then the fluorination is continued at the final temperature until feeding a HF molar amount at least equal to the stoichiometric with respect to the alumina, preferably 1.3 times higher than the stoichiometric, the fluorination being continued until a fluorinated alumina having a fluorine content not lower than 95% of the stoichiometric is obtained.
Preferably the HF flow is diluted with air or inert gas, more preferably air, in volume ratios HF/diluent 0.1:1 to 1:1.
Preferably the thermal gradient is 10xc2x0-90xc2x0 C./hour, more preferably 20xc2x0-50xc2x0 C./hour.
The AlF3 obtainable with the process of the invention has a surface area higher than that obtainable by direct fluorination of alumina at the final temperature.
According to another aspect of the invention, the alumina to be fluorinated is brought to the initial fluorination temperature and can be partially fluorinated at this temperature before starting the gradient up to the final temperature, it is then allowed to fluorinate at the final temperature until a fluorinated alumina with a fluorine content not lower than 95% of the stoichiometric is obtained.
The total pressure has no important effects and one generally operates at atmospheric or at slightly higher pressure, generally of some atmospheres.
It is instead advantangeous that the HF partial pressure is low, especially at the beginning of the fluorination, to moderate the heat development which could locally increase the temperature beyond the limits before mentioned. Indeed two highly exothermic phenomena contemporaneously take place: the reaction between HF and alumina with formation of AlF3 and water; and the hydration of the unreacted HF by water.
To moderate this exotermic process it is sufficient to use HF diluted with an inert gas in the fluorination conditions, for example, air or nitrogen, with the indicated HF/diluent ratio by volume.
A better control of the temperature is achieved also by carrying out the reaction in a fluidized bed and this is the preferred way to carry out the fluorination. In this case the aluminas to be fluorinated have a particle size distribution compatible with the use of fluidized beds.
When the aluminas are in hydrated form, it is preferable to precede the fluorination with a calcination phase in air or nitrogen, at temperatures between 300xc2x0 C. and 400xc2x0 C. This limits the water development during the reaction, which is undesirable especially as it favours the plants corrosion.
The preferred aluminas for the fluorination have pseudoboehmite crystalline structure, surface area of about 300 m2/g. Optionally they can contain silica up to 15% by weight, generally from 1 to 5%.
The aluminas and the aluminum fluorides are characterized by techniques well known to the skilled in the art of solid characterization: the surface area (SA) is measured by nitrogen adsorption according to the BET method. The analytical composition is detrminrd by wet way according to known methods.
The aluminum fluoride obtainable by the process of the invention can advantageously be used as a support for preparing catalysts for the fluorination of halogenated organic compounds, specifically to obtain R-32 CH2F2.